The present invention relates to an optical signal transmission apparatus, and more particularly, to an optical signal transmission apparatus having a dispersion compensation function of compensating a waveform that is distorted due to the dispersion of optical fibers.
Due to rapid increase of data traffic representative on the Internet, an optical communication network for enabling large-capacity communication is required. In order to implement large-capacity communication, a wavelength division multiplexing (WDM) technology is used. In the WDM technology, several tens of wavelengths can be transmitted through one optical fiber. Long-range transmission over several hundreds of kilometers is available by using optical amplifiers or regenerators. In recent years, a wavelength multiplexing transmission apparatus of 10 Gbit/s per one wavelength has been put in practical use.
In designing a wavelength division multiplexing transmission system, a dispersion compensator (DC) is required. The dispersion compensator compensates waveform distortion caused by wavelength dispersion of the optical fiber. For example, a dispersion compensation amount of approximately −1600 ps/nm is required to compensate for wavelength dispersion of a single mode fiber (SMF) having a length of 80 km and wavelength dispersion of +20 ps/nm/km in a communication band of 1.55 μm. In a specific configuration, since the wavelength dispersion is compensated in an optimal waveform, a value different from the above-mentioned value may be selected by considering chirping of an optical signal or non-linear effects of the optical fiber. In the SMF, a zero dispersion wavelength in which the wavelength dispersion is 0 is 1.3 μm. As the type of the fiber, in addition to the SMF, various fibers such as a dispersion-shifted fiber (DSF) in which a dispersion amount in the optical signal wavelength is reduced by shifting the zero dispersion wavelength to 1.55 μm which is a wavelength band of the optical signal, etc. are used.
For such a fiber, a dispersion value of the optical fiber transmission path applied to determine a proper dispersion compensation amount is practically measured or estimated, and the dispersion compensator having the proper dispersion compensation value is mounted on the transmission apparatus. Currently, in a generally used dispersion compensator, a compensation value of a dispersion compensating fiber (DCF) is fixed. However, in the fixed dispersion compensator, there is a problem that plural fibers must be stocked. In a transmission rate of 40 Gbit/s or more, since a spectral width is extended, there is a problem of disagreement (residual dispersion) of batch compensation of a WDM signal by the DCF and the optimal compensation value of each wavelength, the seasonal variation of a fiber dispersion characteristic caused by a change of ambient temperature of the fiber, etc., disagreement of a small dispersion compensation value, or a polarization mode dispersion (PMD) greatly influences a transmission characteristic. In order to solve the above-problems, a variable dispersion compensator is considered.
For example, as the variable dispersion compensator for compensating the wavelength dispersion, a variable dispersion compensation device using a virtual image phase array (VIPA) disclosed in Japanese Patent Application National Publication No. 2000-511655 is used. In addition, a variable dispersion compensation device using fiber Bragg grating (FBG) or etalon is used as the variable dispersion compensator.
Further, for example, as the polarization mode dispersion compensator, a technology disclosed in Japanese Patent Registration No. 3281162 is known. Japanese Patent Registration No. 3281162 discloses that the polarization mode dispersion compensator includes a polarization conversion unit, a group delay application unit, a phase shifter, etc. and compensates waveform distortion caused by the polarization mode dispersion of an optical signal.
As another example, a known technology using a transversal equalizing circuit is disclosed in JP-A-2007-274022. An example of a method of controlling the dispersion compensator is disclosed in JP-A-2002-208892. As an example of a quality monitoring method for control, a method of monitoring an eye opening using an electronic circuit is disclosed in Japanese Patent Registration No. 3995094.
JP-A-H09(1997)-326755 discloses an automatic equalization system that performs equalization or adjustment of a discrimination point of the wavelength dispersion by measuring an error rate, an eye opening degree, a value of Q, and a clock signal level at a reception side.
JP-A-2004-222240 discloses a method and an apparatus of monitoring an optical signal that extract the characteristic of an eye pattern of the optical signal, acquire a factor for deterioration and a deterioration amount of the optical signal, and control a wavelength distortion compensation device.
In the related art, bit error information of a received signal is used to control a variable dispersion compensator. However, the bit error of the received signal may occur due to a factor other than dispersion. For example, noise temporarily increases due to four wave mixing (FWM) caused by a wavelength extension. As another example, since temporary tension is involuntarily applied to a fiber due to device installation work in the vicinity of a device, a loss between optical amplifiers increases and an optical signal-to-noise ratio (OSNR) is deteriorated, whereby the bit error is generated. In this case, when the variable dispersion compensator is controlled only by the bit error information of the received signal, the bit error may unnecessarily increase by being shifted from an optimal compensation value. As a further example, in a case when the fiber is removed during a repairing work, the variable dispersion compensator cannot be controlled while an optical signal is lost, resulting in taking an excessive amount of time to converge into the optimal dispersion compensation value and it takes considerable time to restart the service, at the time of inserting the fiber again. Even if an eye opening is used as information to control the variable dispersion compensator, the same problem occurs.